When I took my first GIS class, I was told that 5% of my time would be spent making maps, while 90% of it would be spent corralling data into usable format and 5% would be spent beating an unresponsive plotter with a cardboard tube. This balance has definitely changed as interoperability has gone from prayer to practice, but I've continued to be frustrated by the time I feel gets lost to a missing shapefile extension or a falsely-defined projection.
For all the fun afforded us by Mapbox's original Tilemill map design platform, it's always been sort of a GIS-y hassle to wrangle data into it. Mind you this is slight, glancing criticism - it's NOTHING compared to getting your geodata to work in Illustrator or any actual GIS platform. But I always found myself wishing I could spend less time racking my brain for where I put that awesome building footprint layer, or trying in vain to find the right ORDER BY syntax for a PostGIS datasource.
Perhaps you've heard, but Mapbox sort of solved my whiny problems. Tilemill 2 is in unsupported alpha, but it's already fulfilling my dream of data-agnostic cartography. The basic idea is that the world - as derived from OpenStreetmap - is some pretty centralizable basedata, so why not just tap the source and style it however you like? Instead of pulling extracts or copies or subsets, Tilemill 2 just gives you the whole damn world, all 330GB-and-counting of it, via super-fast vector tiles (great explanation here). You get to style those tiles with the CartoCSS language, and at that point they're ready for your audience.
Suffice to say, it is A LOT of fun to have the world at your fingertips:
For the moment there's no export or serving option, and it'll be interesting to see what Mapbox does with this tool in its already-robust custom mapping lineup. It's also worth noting that planet.osm is not ALL TEH DATAZ - we'll always need a way to use local or personal geodata. But this is a great leap for an already-impressive platform.
It's liberating to be able to focus on cartography with the building blocks just there at my fingertips.